ABSTRACT
This chapter outlines the history of the introduction and further spread of alien species in cities. It explores global trends in the spread of alien plants and looks at urban-specific factors influencing such introductions. The chapter examines how alien plants spread from cities into adjacent rural areas and discusses urban grasslands as an example of the benefits and disadvantages of alien species. Invasion processes start with the human-mediated transport and introduction of a species to an area beyond its natural range and may further proceed to the establishment and spread of this species in the new range. Human agency is not only decisive for the initial introduction of alien species but often also for subsequent invasion stages. The high abundance of planted and spontaneous alien species in cities leads to questions regarding their function as invasion foci for the countryside. Urban greenspaces usually include grassland areas.
